We report on the photonic band gap phenomenon in the visible range in a three-dimensional dielectric lattice formed by close-packed spherical silica clusters. The spectral position and the spectral width of the optical stop band depend on the direction of light propagation with respect to the crystal axes of opal, and on the relative cluster-to-cavity refraction index n. Manifestations of the photonic pseudogap have been established for both transmission and emission spectra. The stop band peak wavelength shows a linear dependence on n. Transmission characteristics of the lattice have been successfully simulated by numerical calculations within the framework of a quasicrystalline approximation. ͓S1063-651X͑97͒13805-3͔

The decay dynamics and the quenching of the photoluminescence (PL) from Si nanocrystals are investigated. Electron acceptors whose reduction potentials lie below the conduction band (CB) edge of the Si nanocrystals quench the red emission from the Si nanocrystals. The quenching rate constants obtained from Stern-Volmer analyses for 3,5-dinitrobenzonitrile, 4-nitrophthalonitrile, 1,4-dinitrobenzene, 4-nitrobenzonitrile, 2,3-dinitrotoluene, 3,4-dinitrotoluene, 2,4-dinitrotoluene, and 2,6-dinitrotoluene are in the range of 10 6 -10 7 M -1 s -1 . The quenching mechanism occurs via an electron transfer from the CB band of the Si nanocrystals to the vacant orbitals of the quenchers. The PL decay profiles of the Si nanocrystals, in the presence and absence of the quencher, are well described by the stretched exponential decay law. The band gap of the Si nanocrystals estimated from the present study is larger than the PL peak energy. The results are consistent with a quantumconfinement model, where recombination of electrons and holes occurs in a surface state. The ability of nitrotoluenes to quench the PL from Si nanocrystals could be used to develop a sensor based on Si nanostructures for the detection of explosives. IntroductionSilicon nanostructures have stimulated much interest because of their unique properties such as single-electron tunneling, nonlinear optical properties, and visible photoluminescence (PL). 1-7 Studies of Si nanostructures have grown extensively since the discovery of efficient PL from porous Si (p-Si) with the ultimate goals of achieving a complete fundamental understanding of the phenomenon as well as developing potential display devices and chemical-sensor applications. 1-10 Si nanocrystals have optical and PL properties very similar to those of p-Si, and it is generally accepted that they are the emitting chromophores in p-Si. Kinetic quantum confinement appears to be the most reliable model to explain the origin and properties of the PL from Si nanocrystals. The higher energy shift in the PL of Si nanocrystals is attributed to the three-dimensional quantum size effect. The nonradiative recombination process is decreased significantly in the nanocrystal as the electronhole pairs in separate nanocrystals are electrically isolated. However, in both p-Si and Si nanocrystals, the PL properties are related not only to the respective nanocrystalline size but also to the structure and properties of the surrounding medium. The study of these effects is essential for utilizing Si nanostructures in potential device applications.Many organic and inorganic molecules have been shown to efficiently quench the PL from p-Si, and both energy-and electron-transfer mechanisms have been proposed to explain this PL quenching. 11-30 Sailor and co-workers found a number of polar solvent molecules that reversibly quenched the PL from p-Si. 11-15 They also studied the quenching by aromatic molecules and concluded that energy transfer from the p-Si excited state to the triplet levels of the quencher molecules predomi...

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Web-like aggregates of coalesced Si nanocrystals produced by a laser vaporization-controlled condensation technique show luminescence properties that are similar to those of porous Si. The results are consistent with a quantum confinement mechanism as the source of the red photoluminescence (PL) in this system. The oxidized Si nanoparticles do not exhibit the red PL that is characteristic of the surface-oxidized Si nanocrystals. The nanoparticles are allowed to oxidize slowly, and the PL is measured as a function of the exposure time in air. A significant blue shift in the red PL peak is observed as a result of the slow oxidation process. The dependence of quantum size effects on the bonding structure is established by correlating the PL data with the photon-yield electronic structure measurements made at the Advanced Light Source. The results indicate that as the nanoparticles oxidize, the radius of the crystalline core decreases in size, which gives rise to a larger bandgap and consequently to the observed blue-shift in the PL band. The correlation between the PL, SXF, and NEXAFS results provides further support for the quantum confinement mechanism as the origin of the visible PL in Si nanocrystals.